ME31B: CHAPTER SEVEN

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ME31B CHAPTER SEVEN

• DESIGN OF EXTERNAL FACILITIES ONE

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INTRODUCTION

• This chapter deals with structures which are only
  indirectly related to buildings, but which are of
  great importance to the farmer.
• These include roads, culverts, bridges and water
  distribution systems related to farming
  activities.

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7.1 INTRODUCTION TO SIMPLE
ROAD DESIGNS

• Rural access roads range from the simplest earth
  roads to bituminous surfaced highways. However,
  earth roads are normally the only type that can
  be justified for access to farmsteads.
• These roads, designated as unimproved earth
  roads, are generally suitable solely for light
  traffic, up to some dozen or so vehicles per day,
  and they often become impassable in the wet
  season.
• Heavy lorries, which sometimes need to have
  access to farmsteads, should only be allowed on
  this type of road after an adequately long dry
  spell.

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Simple Roads Contd.

• There is no need for actual structural design of
  unimproved roads, but there are some principles,
  which if followed, will produce a reasonably good
  road for the small investment that they justify.

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7.1 Road Location

• A survey to determine the best location for a
  road line starts by identifying areas through
  which the road must pass, for example
• A gap between hills, the best location for a
  river crossing, and points to be linked by the
  road.
• Places to be avoided include soft ground, steep
  slopes, and big rocks. In large scale road
  projects the terrain is viewed from aerial
  photographs, but for smaller projects this is too
  costly and instead an overview of the proposed
  road line must be obtained fromadjacent hills.

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Road Location Contd.

• Such an overview provides valuable information on
  natural drainage, but should always be
  supplemented by a detailed examination on foot.
• Once the points through which the road must pass
  have been established, the road line is laid out
  to run as directly as possible between them

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Road Gradients

• A steep gradient not only slows down traffic and
  limits the load a draught animal can pull,
• The recommended gradient standards for unimproved
  roads differ in different countries, but
  generally, for roads used mainly by motor
  vehicles, the gradient should not exceed 1 in 17
  in flat or rolling terrain, 1 in 13 in hilly
  terrain, or 1 in 11 in mountainous terrain.
• In exceptional cases it may be necessary to have
  steeper gradients, but their maximum length
  should then be limited.

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Road Curves

• A straight road is the shortest distance between
  two points, but this may not be the most
  economical line for a durable, easily constructed
  road which is passable throughout the year.
• Long gentle curves are preferred since there is
  better visibility and less speed reduction
  necessary than on a sharp corner. The minimum
  radius for a horizontal curve is 15m but 30m or
  more is preferable.

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Road Slopes

• Only occasionally will an unimproved road require
  embankments or cuttings, but where it cannot be
  avoided, the side slopes should not exceed 1 in 1
  on well-drained soils. In wet soil it should not
  exceed 1 in 3, i.e. one unit rise in three units
  of horizontal distance.
• These are maximum values and should only be used
  where the depth of the cut or fill is so large
  that to reduce the slope would be too expensive.

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Road Camber

• The camber is the slope of the road surface to
  the sides designed to shed water into the side
  drains.
• A simple earth track has no camber and no side
  drains. But all other roads should have a camber
  of 5 to 7 from the middle of the road, thus
  shedding water into both side drains. In deep
  cuts (where the road is dug into a hill side) or
  on sharp curves, the camber is designed to drain
  water from the whole surface inwards toward the
  cut or to the inside of the curve.

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7.1.6 Cross Section of a Simple Earth Track

• The simplest earth track is obtained by merely
  clearing vegetation and stones from the natural
  soil surface.
• It may run between fields within a farm, from the
  main road to a farmstead or between small
  villages where the traffic volume is very low.
• Earth tracks are based on single lane traffic in
  one pair of wheel tracks, but vegetation should
  have been cleared wide enough to allow for two
  small cars to meet.

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Cross-Section of a Simple Earth Track
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Cross Section of an Upgraded Earth Road

• These roads may be used to connect rural
  market centres and villages where the traffic
  volume is 10 to 20 vehicles per day including
  some heavy lorries in the dry season.
• Generally the only affordable surface material
  is the soil found on the line of the road or in
  its immediate surroundings.
• The bearing capacity of the road depends on
  the type of soil and the prevailing climatic
  conditions.

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Cross Section of an Upgraded Earth Road Contd.

• The road is constructed by digging out soil from
  the sides and throwing it on the road until the
  cross section illustrated in Figure below is
  obtained.
• The 30 cm difference in level between
• the road surface and the bottom of the side
  drains, combined with the camber of the road
  surface, will ensure a much drier roadway with
  higher carrying capacity than the simple earth
  track.

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Methods For Improving Earth Roads

• Gravelling Reduces the risk of mud forming
• Paving with pit run or with pitch
• Grassing to improve their strengths
• Constructing side drains
• These improvements are common on earth roads in
  Trinidad including Nariva Swamp

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7.1.8        Road Construction

• When the land has been surveyed and the most
  feasible road line has been found, the centre
  line of the road is set out with pegs inserted at
  15 to 20m intervals and tall enough to be clearly
  visible.
• Additional pegs may be installed to mark the
  width of the roadway, side drains and the area to
  be cleared. 
• Stumping and Clearing
• To construct a simple earth road, trees and rocks
  must be cleared from the road line and well back
  from the road so that sun and wind can dry the
  road surface.

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Road Construction Contd.

• If the objective is to construct a high-level
  earth road, the work will continue with the
  construction of side drains.
• Construction of Side Drains
• Using wooden pegs and string as a guideline, the
  edge of the road should be established 1.8 to
  2.0m from the center line.
• On roads with no cross-fall, side drains are dug
  out of either side to a depth of 150 mm and half
  the width of the roadway.
• All soil thus dug out is thrown on to the road
  and spread to form an even road surface with
  correct camber.

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7.1.8       Road Maintenance

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• The most important maintenance job on any type of
  earth road is to ensure that all drains work
  properly and that additional drains are installed
  wherever it becomes necessary.
• Secondly, rutted wheel tracks should be filled in
  with soil from outside the road bed.
• If the road surface becomes badly deteriorated
  it will be necessary to resurface the road by
  adding more soil from the side drains.

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7.1            Culverts 

• Where the road crosses a natural water way, a
  culvert or bridge should be built.
• Culverts are best suited for streams with steep
  banks, since their construction requires some
  difference in height between the level of the
  road surface and the bed of the stream.

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Culverts Contd.

• Culvert construction consists of the following
• 1 The actual culvert (one or more pipes), which
  carries the water under the road.
• 2 The embankment, which carries the road across
  the water course.
• 3 Wing walls, which protect the embankment from
  flood water and direct the flow into the culvert.
• 4 The apron at the discharge end, which prevents
  erosion of the stream bed.

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Culverts Contd.

• The normal water flow is carried by the culvert,
  but large flows of storm water are allowed to
  flow over the top of the embankment.
• Concrete pipes, 400 to 900 mm in diameter, are
  often used for culverts.
• The diameter and number of pipes is determined by
  the expected water flow.
• Alternatively corrugated steel pipes or masonry
  work in burnt bricks, concrete blocks or stone
  may form the culvert.

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Culverts Concluded

• Where concrete pipes have been used for a
  culvert, the embankment must provide for a soil
  cover above the pipe to a depth at least equal to
  the diameter of the pipe in order to sufficiently
  protect the pipes from the load of heavy
  vehicles.
• The beams in the ceiling of a square shaped
  culvert with masonry walls may be designed to
  carry the load of vehicles, thus reducing the
  need to spread the load in the embankment by a
  soil cover.

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7.1  Simple Bridges

• The ideal site for a bridge is where the river is
  narrow and the banks are solid.
• The bridge should be designed to interfere as
  little as possible with the natural flow of
  water.
• The highest level, which the river is known to
  have reached, is determined and the bridge
  designed to give at least 0.5m clearance above
  that level.

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Components of Simple Bridges

• 1. Abutments, the structures provided to
  strengthen the stream banks and adequately
  support the shore end of the road-bearing beams.
• They can be constructed of concrete, masonry work
  (stone, brick, concrete blocks) or timber. The
  lower part of the abutments will normally require
  wing walls to protect them from the action of the
  stream.
• Intermediate supports installed where the stream
  is too wide to be bridged in a single span.
  Timber trestles, masonry piers and reinforced
  concrete columns are the most common types of
  support.

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Components of Simple Bridges Contd.

• 2. Road-bearing beams that carry the weight of
  the roadway and traffic between abutments and
  any intermediate supports. Simple bridges have
  road-bearing beams consisting of round or sawn
  timber or universal steel beams spaced about 600
  mm center-to-center across the roadway.
• For example, a bridge 3.0m wide requires 6 beams
  and a bridge 3.6m wide, 7 beams etc.
• The beams are usually designed as simple beams
  supported at the ends.

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Components of Simple Bridges Contd.

• 3. Decking or flooring, which make up the road
  surface on the bridge.
• Where poles or other rough materials have been
  used for decking a smoother surface can be
  obtained by putting planks along the bridge for
  the wheel tracks.
• The decking should be strong enough to spread the
  load from one wheel over at least two
  road-bearing beams. Wooden decking should never
  be covered with soil, since that will increase
  decay and disguise any weakness in the bridge.

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Components of Simple Bridges Concluded

• 4. Curbs made from poles or pieces of timber
  should be secured to the edges of the decking.
  Curbs will reduce the risk of vehicles slipping
  over the edge and will also, if positioned over
  the outer road-bearing beams and well secured to
  them, contribute to the strength of the bridge.
• 5. Rails along the edges of the bridge for
  safety.

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Concluding Remarks About Simple Bridges

• The bridge must be designed to carry the weight
  of the members of the bridge (dead load) and the
  weight of any traffic moving across it (moving
  load).
• In order to simplify calculations, the moving
  load is often converted to an equivalent live
  load by multiplying it by 2.
• When a heavy lorry moves across the bridge, the
  bridge will carry concentrated loads from the
  wheels with spacing equal to the wheelbase and
  tread-width.

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Concluding Remarks About Simple Bridges Contd.

• In a bridge of short span the largest bending
  moment in the road-bearing beams will occur when
  the back wheels which carry the greatest weight
  are at the centre of the span and will be
  determined by half the weight on one wheel, since
  the decking is designed to distribute the load to
  at least two beams.
• In a bridge of longer span where both front and
  rear wheels may be on the span at the same time,
  the maximum bending moment will occur when the
  centre of the wheel base is a short distance from
  the centre of the span.  

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Concluding Remarks About Simple Bridges

• In addition to bending, shear may have to be
  considered in short spans, and deflection for
  long spans.
• Where bridges are constructed with rough
  materials under unfavourable conditions, a
  larger factor of safety should be used.

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7.1   DESIGN OF WATER DISTRIBUTION SYSTEMS

• 7.4.1 Demand and Consumption of Water
• Consumption is the amount of water used in
  reality e.g. in domestic needs. It rises to
  demand according to water supply improvements.
• Demand is the amount of water that would be used
  by consumers if available, under specific
  conditions of price, quality and others.

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Uses of Water

• Water is normally used for domestic, tourist,
  fire-fighting, industrial, agricultural (mainly
  irrigation) and hydro-electricity.
• Typical domestic water use in the Caribbean is
  given in Table 7.1 below.

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Agricultural Water Use

• The agricultural consumption is mainly the crop
  water requirements, usually higher than human
  needs. In the Caribbean region, this ranges from
  1 to 1.5 m (gross) per crop per season. This
  amounts to about 10 to 15 million litres per crop
  per season.
• Another form of agricultural consumption is
  livestock requirement, which can be about 64
  litres per hr per day for cattle.

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Water Contd.

• Design should be based not on present water
  demand but on future demand estimation which is
  normally obtained by extrapolation.

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7.4.2   Storage and Distribution of Water

• Service Reservoirs Storage in water supply
  network 
• 7.4.3.1 Purposes for Storage
• (i) To balance supply and demand
• (ii) Protection against breakdown
• (iii) To provide a static head for gravity
  running
• (iv) Water treatment.

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Siting and Capacity of Reservoir

• 7.4.3.2 Siting of Reservoir It should be sited
  as close as possible to point of use within
  constraints of available relief. This is to
  reduce the pipe cost due to the higher discharge
  from storage to points of use.
• 7.4.3.3 Capacity of Reservoir Inflows should be
  kept fairly even. Outflows can be peaked.
  Storage is used to balance uniform inflow and
  non-uniform outflows. If inflow is greater than
  outflow, then water is getting into storage and
  if outflows is greater than inflows, water is
  coming out of storage.

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7.4.3.4  Pipes

• There are three categories of pipes
• (i)MainsTrunk - not tapped and Distribution
  Mains supply water. They have relatively large
  diameter and are used for conveyance and
  distribution. Materials used include cast iron,
  spun iron, asbestos, cement, or steel.
• (ii) Service Pipes Individual supply lines to
  farms, houses and hospitals or standpipes.
  Materials used include copper, steel, plastics
  (PVC or polyethylene).
• (iii)  Plumbing Pipe work within the building

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7.4.3.4   Pressure Classes of Pipes

• There are three important pressures associated
  with pipes.
• (i) Work Test 2 to 3 times the working
  pressure. It is the pressure used to test
  manufactured pipes.
• (ii) Maximum Field Test One and half times the
  working pressure. The specified design pressure
  should be tested in the field.
• (iii) Maximum Working Pressure Maximum pressure
  derived in the field. There are three classes of
  maximum working pressures e.g. polyethylene Class
  B- 6 bars, Class C - 9 bars and Class D 12
  bars.

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7.4.3.4  Pipeline Design

• The selection of pipes is an economic tradeoff
  between large diameter which will give high
  capital cost and low friction losses and low
  pumping costs (if there is pumping) OR small
  diameter, which will involve low capital cost,
  more head losses and more pumping cost.
• Energy cost is a function of head losses while
  pipe cost is a function of diameter.

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Allowable Head Losses

• (i) Allow 1 m (for big pipes) to 10 m (small
  pipes) head loss per 1000 m of mainline
• (ii) Using velocity as criteria as head loss
  effects is related to velocity.
• Normal practice in water supply for irrigation is
  to keep velocity within 0.6 to 1.5 m/s. Above
  that, there can be water hammer or high rates
  of corrosion. Water hammer is transient high
  pressure waves due to rapid valve closure. Below
  0.6 m/s, there may be silting or sediment
  deposition.
• Pipe diameter can be chosen using head losses and
  velocity using charts or equations.

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7.4.3.4  Pipe Layout Types of Distribution
Systems 

• (i) Individual Pipes Connects two points in the
  distribution system say from a reservoir to the
  point of use.

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Example 1 A reservoir (Figure 7.7) is situated
65 m vertically above some farm buildings. The
length of pipe required to lead water from the
reservoir is 750 m and the pressure required at
the buildings is 30 m head. Rate of flow
required is 2 m3/h (2000 litre/hr).
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Solution

•  If the head available due to the height of the
  reservoir is 65 m, and the pressure head needed
  at the buildings is 30 m, the head available for
  overcoming friction is 65 30 35 m being the
  difference in head between the ends of the pipe.
  
• The equivalent length of the pipe is
• Actual length (750 m) 10 (75 m)
• 825 m
• Plus (say) 1 tap 2 stop taps 135 m
• Total 1060 m

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Solution Concluded

• The hydraulic gradient is Pressure difference/
  equivalent length 35/1060 1/30
• Since the maximum head is 65 m, a Class C (9 bar
  or 90 m) pipe is required, and referring to Chart
  provided, it can be seen that a 32 mm nominal
  (internal) diameter Class C low density polythene
  pipe would satisfy these requirements.
• Velocity is about 0.8 m/s which is acceptable
  (within 0.6 and 1.5 m/s).

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Chart
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(ii) Branching System

• The advantages are relatively few joints and the
  system is easy to build and design.
• The disadvantages are that sediments may
  accumulate at dead ends of the pipe. Secondly,
  it there is pipe bursts, a total cut off for zone
  beyond failure results.
• This means that in case of bursts, the system
  will be cut off.
• Also there is limitations in adding to the system
  beyond a certain point.
• Because of these disadvantages, branch system is
  used in small community projects.

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Solution Computation Table
Pipe Sect. Flow (m3/h) Length (m) Pipe Dia mm Head Loss (m/100 m Flow Vel m/s Head Loss (m) Elev. of hydr. Grade (m) Ground level elev (m) Press Head (m) Rem.
AB 2.9 700 32 3.3 0.85 23 A 260 B 237 189 46 O.K
BC 0.5 825 19 1.6 0.5 13 B 237 C 224 219 5 Just O.K
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Explanation of Table

• The average of the maximum and minimum pressure
  required at A is 41 m.
• If you subtract the minimum pressure needed at B
  (5 m) from 41 m, you get 36 m.
• Since the length of the pipe is 700 m, the
  hydraulic head loss is 36/700 0.051 5/100
  1/20.
• With the discharge of 2.9 m3/h and head loss of
  1/20, the next higher diameter of pipe is 32 mm
  from the chart.

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Chart
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Explanation of Table Contd.

• With now 32 mm diameter pipe chosen in column 4
  of the Table, and the same flow rate, the actual
  head loss is now 1/30 from the chart which is 3.3
  m/100m as shown in column 5.
• The flow velocity is about 0.85 m/s which is
  acceptable.
• The head loss is now (3.3 x 700)/100 23 m. At
  A, the elevation of the hydraulic grade line is
  now 41m ground elevation (219 m) 260 m.

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Explanation of Table Contd.

• For B, it is 260 minus the head loss (23 m) which
  is 237 m.
• The ground elevation at B is 189, so the pressure
  head of water is 237 189 48 m which is
  adequate.
• For Pipe BC, the design flow is 0.5 m3/h. The
  hydraulic grade line at B is still 237 m and the
  elevation at C is 219 m.
• The hydraulic grade line required at C is 219 m
  plus 5 m head of water, making a total of 224 m.

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Explanation of Table Concluded.

• The hydraulic gradient from B to C is then (237
  224)/825 0.016 which is 1.6/100 1/60.
• With hydraulic gradient 1/60 and the flow rate
  of 0.5 m3/s, the diameter of pipe from the Chart
  is exactly 19 mm.
• The velocity is 0.5 m/s which is barely
  acceptable.
• The head loss is 0.016 x 825 m 13 m.
• The hydraulic grade line at C is therefore 237 m
  13 m, which is 224 m. This will give the
  pressure head of 5 m required at C.

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(iii)Grid Pattern/Looped Network

• Interconnected pipes water reaches a point from
  a number of directions.
• The advantages are that there will be no
  stagnation i.e. no dead ends and during repairs
  (pipe burst), there will be no need for complete
  cutoff.
• Only some parts of the system will be cut off.
  There are also more even pressures throughout the
  system.
• The disadvantages are that the designs are more
  complicated and there are more pipes and more
  fittings.

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Pipe Network Analysis Using the Hardy Cross
method.

• The Hardy Cross system is used for water flow
  analysis in a more complex system
  than the dead end system.
• There are two principles In any closed loop

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Procedure For Analysis 

• 1. Assign assumed flows to each pipe segment in
  network such that at each junction
  
• 2. Calculate hf for each pipe using for example
  Hazen Williams equation 
• hf 10.67 CH -1.85 D- 4.87 Q1.85 L
• Where hf is head loss (m), CH is roughness
  coefficient of pipe material D is diameter of
  pipe (m), Q is water flow rate (m3/s) and L is
  length of pipe (m).

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Procedure Concluded

• For any pipe that occurs twice, do the
  
• correction for the two loops.
•  
•  
•  
• BC occurs twice.
• 8. Repeat from step 2 until desired accuracy is
  obtained.

B
E
A
D
F
C
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Example
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Solution

• ABDE is one loop as shown above and BCD is the
  second loop.
• Note that the clockwise water flows are positive
  while the anti-clockwise ones are negative.
• Positive and negative flows give rise to positive
  and negative head losses respectively

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Final Water Flows